1. Field of the Invention
This invention relates generally to an electronic device for indicating the presence of electromagnetic interference (EMI). More specifically, the present invention relates to a stability indicator in a electronic device for detecting and indicating the presence of a high electromagnetic field in the immediate vicinity of the device. In particular, the present invention relates to a fail-safe detection circuit embodied in a tympanic thermometer that prevents the taking of a temperature reading in the presence of high EMI event and displays a warning.
2. Prior Art
The diagnosis and treatment of many body ailments depends upon an accurate reading of the internal or core temperature of a patient's body, and in some instances, upon comparison to a previous body temperature reading. For many years, the most common way of taking a patient's temperature involved utilization of Mercury thermometers. However, such thermometers are susceptible to breaking and must be inserted and maintained in the rectum, axilla or mouth for several minutes, often causing discomfort to the patient.
Because of the drawbacks of conventional Mercury thermometers, electronic thermometers were developed and are now in widespread use. Typically, such electronic thermometers have a probe connected by wires to a remote unit containing electronic circuitry. The probe is sheathed in a protective, disposable cover before being inserted into a patient's mouth, axilla or rectum. Using predictive techniques, the patient's temperature reading is taking a significantly shorter time period, for example thirty seconds, compared to several minutes required for conventional Mercury thermometers. Also, the electronic thermometer in some instances provide more accurate temperature readings than Mercury thermometers.
Although electronic thermometers provide relatively more accurate temperature readings than Mercury thermometers, they nevertheless share many of the same drawbacks. For example, even though electronic thermometers provide faster readings, a half minute must still pass before an accurate reading can be taken. Finally, electronic thermometers must still be inserted into the patient's mouth or rectum which can cause patient discomfort or adversely affect temperature reading accuracy if the probe is moved during the course of measurement.
Tympanic thermometers provide nearly instantaneous and accurate reading of core temperature without the undue delay attendant with other thermometers. The tympanic membrane is generally considered by the medical community to be superior to oral, rectal or axillary sites for taking a patient's temperature. This is because the tympanic membrane is more representative of the body's internal or core temperature and more responsive to changes in core temperature. Tympanic thermometers, those thermometers that sense the infrared emissions from the tympanic membrane, offer significant advantages over Mercury or conventional electronic thermometers.
Recent efforts to provide a method and apparatus for measuring body temperature inside the tympanic membrane have produced several excellent tympanic thermometers. For example, U.S. Pat. No. 5,293,877 to O'Hara et al. provides for a tympanic thermometer that measures internal body temperature utilizing the infrared emissions from the tympanic membrane and within the ear canal itself, and is herein incorporated by reference in its entirety.
The tympanic thermometer of O'Hara et al. is comprised of a probe unit that has a handle and a probe head body terminated in a probe tip which is inserted into the external ear canal. The handle houses a circuit board that controls the operation of the thermometer and a display that displays temperature readings and other information. The probe head body is attached to the distal end of the circuit board and houses a seal assembly, optical waveguide tube, infrared filter and thermopile detector. The probe head body further includes a first bore in communication with a second narrower second bore. The distal end of the first bore forms a tip with an opening thereto for passing infrared emissions from the tympanic membrane into the probe head body. The infrared filter is mounted in the opening and rejects unwanted emissions while the optical waveguide tube conducts the infrared emissions to the thermopile detector located at the proximal end of the tube. In order to prevent contamination from entering the probe head body, a seal assembly is also provided that furnishes a watertight barrier against liquid and debris from entering through the interface between the probe tip and the infrared filter.
The user operates the thermometer by inserting the probe tip into the patient's ear canal and depressing the SCAN button once the probe tip is properly seated inside the ear canal. At this point, infrared emissions from the tympanic membrane are filtered through the infrared filter and conducted by the optical waveguide tube until detected by the thermopile detector. Actuating the SCAN button also alerts the microcomputer that the tympanic comparative computation algorithm should commence. Once the microcomputer is alerted, it starts acquiring the thermopile output level at a rate of approximately seven times per second and stores the maximum temperature reading for display to the user. However, being electronic devices tympanic thermometers still suffer from the effects of nearby sources of electromagnetic interference.
It is well known in the electronic art that circuits and other electronic devices, like tympanic thermometers, may be adversely affected by the presence of electromagnetic interference. By definition, electromagnetic interference is an unwanted electromagnetic signal which may degrade the performance of an electronic device by creating undesirable voltages or currents in the device's circuitry. The cause of an electromagnetic interference problem is usually an unplanned coupling between an electromagnetic source and a receptor by means of a transmission path. Such a transmission path may be conducted or radiated. Conducted interference occurs by means of metallic path wherein an electrical device is connected to a power source while radiated interference occurs by means of near- and far-field electromagnetic field coupling. It is the latter type of interference that poses the most trouble to medical devices in hospitals.
Sources of radiated interference usually originate from transmitters and other similar types of communication equipment, especially cellular phones. In a hospital or clinical environment, almost all electronic medical devices are designed to sufficiently suppress or reduce most interference generated by an electromagnetic signal in that particular environment. However, mobile radio transmitters and cellular phones radiate particularly high levels of electromagnetic interference, especially where in close proximity to an electronic medical device. In addition, many common medical devices emit EMI and can themselves pose a threat to other medical equipment. These potential sources of EMI include diathermy units, magnetic resonance imaging (MRI) systems, lasers and cauterizers.
In particular, the front-end circuitry of tympanic thermometers which senses the core body temperature of a patient is especially susceptible to EMI radiated by these aforementioned sources. A doctor, for example, operating a cauterizer next to a nurse who is taking a temperature reading using a tympanic thermometer will generate sufficient EMI from the cauterizer to adversely influence the accuracy of the temperature reading taken by the thermometer's probe. The problem faced by medical practitioners in using tympanic thermometers is that the practitioner does not know when an inaccurate temperature reading has been taken in the presence of a high electromagnetic environment because there is usually no outward indication from the tympanic thermometer that the temperature reading is in error. Further, conventional methods of suppressing electromagnetic interference, like shielding and filtering, are insufficient in addressing the problem because the thermometer's circuitry cannot be properly shielded from a close EMI source since shielding merely attenuates, but does not eliminate the interference.
As of yet, nothing in the prior art has addressed the problem of developing an electromagnetic interference detector that gives an outward indication that a tympanic thermometer is being operated in a high electromagnetic environment and prevents the thermometer from taking a temperature reading.
Therefore, there exists a need in the medical device art for a stability indicator that detects and warns the practitioner of the presence of high EMI while disabling the apparatus from taking a temperature until the EMI event has passed.